One particular Windoze XP machine at work keeps falling off the network, possibly relating to an infection by W32.Downadup that we can't seem to get rid of.
Thanks to

http://perljam.net/notes/microsoft-windows-network-the-network-is-not-present-or-not-started/

At least I've now figured out how to get it talking again without having to restart it.
To wit:
"The answer is pretty easy- run services.msc and enable Workstation."

A pox upon the idiots who refuse to convert this place to Mac OS X.